raid size calculator

What This RAID Size Calculator Tells You

This RAID size calculator helps you quickly estimate how much usable storage you get from a set of drives. It also shows how much space is consumed by parity or mirroring overhead, plus the expected storage efficiency. If you are planning a NAS, home lab, backup target, or small business file server, this is a practical way to compare RAID choices.

The calculator assumes all active drives in the array are identical in size. In real systems, mixed-drive arrays typically behave as if every drive were the size of the smallest one.

How to Use the Calculator

• Select a RAID level (0, 1, 5, 6, or 10).
• Enter the total number of installed drives.
• Enter capacity per drive and choose GB or TB.
• Add optional hot spare count if you plan to reserve one or more drives.
• Click Calculate to view raw capacity, usable capacity, overhead, and fault tolerance notes.

RAID Levels Explained

RAID 0

RAID 0 stripes data across disks for performance and full capacity usage. It has no redundancy. If any single drive fails, the full array is typically lost. This option is best for speed-focused temporary workloads, not critical data.

RAID 1

RAID 1 mirrors data. Usable capacity equals one drive's worth when all drives are in a single mirror set. It provides strong redundancy but lower storage efficiency.

RAID 5

RAID 5 uses single distributed parity. One drive worth of capacity is reserved for parity across the array. It can survive one drive failure, offering a balance of efficiency and protection.

RAID 6

RAID 6 uses dual distributed parity. Two drives worth of capacity is reserved for parity. It can survive two simultaneous drive failures and is often preferred for larger arrays.

RAID 10

RAID 10 combines mirroring and striping. You get excellent performance and robust resilience, but usable capacity is roughly half of active raw capacity and requires an even number of active drives.

Capacity Formula Reference

• Raw Installed Capacity = total drives × per-drive size
• Active Array Drives = total drives − hot spares
• RAID 0 Usable = active drives × per-drive size
• RAID 1 Usable = 1 × per-drive size (single mirror set model)
• RAID 5 Usable = (active drives − 1) × per-drive size
• RAID 6 Usable = (active drives − 2) × per-drive size
• RAID 10 Usable = (active drives ÷ 2) × per-drive size

Important Planning Notes

• RAID is not backup. It improves availability, not protection from deletion, ransomware, or silent corruption in every case.
• Filesystem overhead exists. Actual formatted space will be slightly lower than estimated usable capacity.
• Rebuild risk grows with disk size. Larger arrays and larger drives can take longer to rebuild after failure.
• Use matching drives when possible. Performance and reliability are easier to manage with consistent models and firmware.
• Monitor health metrics. SMART checks, alerts, and regular scrub jobs help catch problems early.

Which RAID Level Should You Choose?

There is no universal best RAID level. Pick based on workload, uptime goals, and budget:

• Media editing scratch space: RAID 0 (with external backup).
• Simple mirrored storage: RAID 1.
• General-purpose NAS: RAID 5 or RAID 6.
• High IOPS + resilience: RAID 10.

Final Thoughts

A RAID size calculator is one of the fastest tools for making better storage decisions before you buy hardware. Use it early in planning to avoid surprises in usable capacity and to align array design with your fault-tolerance target. Then pair your RAID plan with a real backup strategy (for example, 3-2-1 backups) for complete data protection.